Field performance of tractor draught control systems

The depth control performance of the draught control systems of two currently produced tractors was studied when ploughing over a single sinusoidal surface undulation of wavelength 30 ft and amplitude 4 in. Each tractor carried a mounted three-furrow mouldboard plough and the variation in the depth of the rear furrow was measured at speeds from 1½ to 6 mile/h in medium and fast response. All the trials were carried out at a set depth of 6 in. In all cases the rear furrow tended to run shallow at or soon after the bottom of the undulation and to run deep near the crest. Errors in depth increased with forward speed; at speeds exceeding 4 mile/h with one tractor and 6 mile/h with the other the plough came out of the ground. A comparison made with a mounted plough with depth controlled by depth wheel showed it to give superior performance at 2 mile/h; at higher speeds errors in depth increased more rapidly than with draught control. A kinematic analysis of the motion of each tractor-plough combination crossing the undulation indicated that the errors in depth resulted from the geometrical constraint imposed on the plough and tractor by the ground undulation. Top link sensing of the control signal acts to assist the ground geometry in producing errors.